JP2006154163A - Hologram recording device, hologram reproducing apparatus, hologram recording method, and hologram reproducing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hologram recording and reproducing device and method capable of making the degree of multiplexing of holograms by an angle multiplexing system higher than heretofore without degrading crosstalk within a limited movable range of a reflection mirror to change the incident angle of reference light and capable of making the movable range of the reflection mirror narrower than heretofore when the degree of multiplexing of the hologram is not raised. <P>SOLUTION: At the time of multiplex recording the holograms to the same region by changing an incident angle of reference light on the hologram recording medium in recording the interference fringes of the reference light and signal light to the hologram recording medium, the angle pitches of the incident angle of the reference light are changed like Δθ<SB>1</SB>, Δθ<SB>2</SB>, ..., Δθ<SB>m-1</SB>, according to the incident angle of the reference light to the minimum values within the ranges where the crosstalk is not degraded. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a hologram recording / reproducing apparatus and method for multiplexing and recording interference fringes of reference light and signal light on a hologram recording medium by an angle multiplexing method, and more particularly, to setting an angle pitch of an incident angle of reference light on a hologram recording medium. .

  Conventionally, during recording of hologram data storage, a hologram recording medium is arranged in an area where two light beams of signal light and reference light interfere, and the interference fringes of the two light beams are recorded on the hologram recording medium (for example, non-patent). Reference 1). At that time, in order to improve the recording density on the hologram recording medium, multiplex recording of holograms by various multiplexing methods such as an angle multiplexing method, a shift multiplexing method, a speckle multiplexing method, and a phase code multiplexing method is performed. Yes.

  In any of the multiplexing systems, the basic configuration of the hologram recording / reproducing apparatus can be considered to be almost the same except that the type of the reference light wavefront that determines the multiplexing principle is different. There is a recording / reproducing apparatus. This illuminates the hologram recording medium with two light beams of signal light and reference light, but each time the signal light is spatially modulated (intensity modulated) on the data page displayed on the spatial modulator (SLM), hologram recording of the reference light is performed. The hologram is multiplexed and recorded in the same recording area by changing the incident angle to the medium. What is recorded on the hologram recording medium is a Fourier image of a two-dimensional bit data image (data page) displayed on the SLM. The reproduction signal is a data page image obtained by inverse Fourier transform of diffracted light, and is captured by an image sensor such as a CCD, and is taken into a computer as a bit pattern.

  The feature of multiplex recording by this angle multiplexing method is that multiple holograms are multiplexed in the same area by reflecting the reference light beam, which is a plane wave, with a rotating mirror (or galvanometer mirror) and changing the incident angle to the recording medium. There is something you can do. This is possible because of the angular selectivity as a feature of the volume hologram.

  Here, the angle selectivity means that when reproducing a single hologram, the diffraction efficiency shows a behavior according to a Sinc function with a deviation amount of the reproduction reference light from the recording reference light angle as a variable. That is, when the recording wavelength of the hologram is λ, the thickness of the recording medium (recording material) is T, and the incident angles of the signal light and the reference light are θo = θr = θ, the diffraction efficiency η is the angle change amount Δθ of the reproduction reference light. For the following,

η (proportional) sinc ² (2T (Δθ) sinθ / λ) (1)

  For example, the relationship between the diffraction efficiency of a hologram recorded at a recording wavelength of 532 nm, a recording medium thickness of 1 mm, and an incident angle of signal light and reference light of 20 degrees is plotted in FIG. It behaves like the drawn characteristic curve.

In addition, the amount of angular deviation of the reproduction reference light is gradually increased from zero, and the amount of change in angle when the diffraction efficiency first becomes zero is the wavelength λ of the light source used for recording and the thickness of the recording medium (recording material). T, determined by the incident angle θo of the signal light and the incident angle θr of the reference light,
Δθ = (Δθ) _B = λ / T (sin θr + sin θo · cos θr / cos θo) (2) However, all the angles shown so far are values in the recording material.

  Consider a case where the incident angle of the reference light changes during holographic recording of holograms. For example, when the refractive index of the photopolymer is 1.5, the incident angle of the signal light in air is 15 degrees, and the incident angle of the reference light is changed from 35 degrees to 65 degrees, Δθ can be obtained from Equation 2. With the characteristics shown in FIG. 8, Δθ decreases as the incident angle increases.

The table in FIG. 9 shows changes in the reference light incident angle when a plurality of data pages are recorded in one recording area by angle multiplexing. The change in the reference light incident angle is constant at Δθ. Therefore, the movable range of the reference light is expressed as θtotal = (M−1) × Δθ (3).
IBM J.RES DEVELOP VOL 44 NO.3 MAY 2000 `` Holographic data storage ''

  As described above, in the multiplex recording by the conventional angle multiplexing method, the minimum amount of change (angle pitch) Δθ of the reference light that secures the angle selectivity varies depending on the incident angle of the reference light as shown in FIG. Nevertheless, since it is constant, Δθ that minimizes crosstalk among all the multiplexed holograms must be selected, and the worst (large) value is adopted as the angular pitch. Therefore, when the movable range of the reference light reflecting mirror is limited, there is a problem that the number of holograms that can be multiplexed decreases.

  The present invention has been devised in view of the above circumstances, and an object of the present invention is to provide a multiplicity of holograms by an angle multiplexing method within a limited movable range of a reflection mirror that changes an incident angle of reference light. To provide a hologram recording / reproducing apparatus and method capable of reducing the movable range of the reflecting mirror in comparison with the conventional method when the hologram multiplicity cannot be increased without increasing the crosstalk. .

  In order to achieve the above-mentioned object, the present invention changes the incident angle of the reference light to the hologram recording medium and records data in the same recording area when recording interference fringes of reference light and signal light on the hologram recording medium. Is a hologram recording apparatus that multiplex-records reference light incident angle control means for changing an angle pitch of the incident angle of the reference light during the multiplex recording.

  Further, the reference light incident angle control means of the present invention is characterized in that the angle pitch of the reference light incident angle is changed according to the incident angle of the reference light.

  The present invention also provides a hologram reproducing apparatus for reproducing multiplexed recording data by changing the incident angle of the reproduction reference light to the hologram recording medium and irradiating the same area of the hologram recording medium with the reproduction reference light. Reproducing reference light incident angle control means is provided for changing the angle pitch so that the incident angle becomes the same as the incident angle of the reference light at the time of recording.

  As described above, according to the present invention, in the hologram recording apparatus using the angle multiplexing method, the angle pitch of the reference light incident angle at the time of multiple recording, which has been conventionally constant, is changed in accordance with the incident angle, so that crosstalk does not occur. The optimum value, for example, the minimum angle pitch of the reference light that ensures angle selectivity, can increase the multiplicity when the reference light movable angle is the same as compared to the case where the angle pitch is uniformly determined. When the multiplicity is the same, the reference beam movable angle can be reduced.

According to the present invention, when the interference fringes of the reference light and the signal light are recorded on the hologram recording medium, the hologram is multiplexed and recorded in the same recording area by changing the incident angle of the reference light to the hologram recording medium. At this time, by changing the angle pitch of the incident angle of the reference light at the time of multiplex recording as an optimum value corresponding to the incident angle (for example, the minimum angle pitch of the reference light to ensure angle selectivity) Within the reference light reflecting mirror movable range, the hologram multiplicity can be increased efficiently without deteriorating the crosstalk.
Further, when the hologram multiplicity is not increased, the movable range of the reference light reflecting mirror can be reduced, so that the optical components of the reference optical system, particularly movable components such as a mirror can be reduced in size.
Further, since the amount of change in the incident angle of the reference light becomes small, the change in the power density of the reference light also becomes small, and therefore the change in the recording / reproducing conditions itself becomes small, so that the apparatus can be simplified.
Furthermore, since angle scheduling is performed to change the angle pitch of the incident angle of the reproduction reference light as an optimum value according to the incident angle during reproduction, an angle suitable for reproduction of each data page even if the hologram recording medium contracts. Correction can be performed, and data reproduction with a good S / N becomes possible.

  Within the limited movable range of the reflecting mirror that changes the incident angle of the reference beam, the multiplicity of holograms by the angle multiplexing method can be increased from the conventional level without deteriorating crosstalk, or the multiplicity of holograms is not increased. In this case, when the interference fringes of the reference light and the signal light are recorded on the hologram recording medium, the angle of incidence of the reference light on the hologram recording medium is changed for the purpose of narrowing the movable range of the reflection mirror. This was realized by changing the angle pitch of the incident angle of the reference light at the time of multiplex recording as an optimum value that does not cause crosstalk in accordance with the incident angle when multiplex recording of holograms in the same recording area.

  FIG. 1 is a block diagram showing a configuration of a hologram recording / reproducing apparatus according to an embodiment of the present invention. The hologram recording / reproducing apparatus includes a laser light source 1, a shutter 2, a half-wave plate 3, a deflecting beam splitter (PBS) 4, a spatial filter 5, a collimating lens 6, a mirror 7, an angle rotating mirror 8, a shutter 9, a mirror 10, and a spy. The image pickup device 17 is composed of a Schal filter 11, a collimator lens 12, a mirror 13, an SLM 14, a signal light lens 15, a reproduction light lens 16, and a CCD or CMOS. Recording and playback are performed using the angle multiplexing method.

  FIG. 2 is a block diagram showing the configuration of the control system of the hologram recording / reproducing apparatus of the present embodiment shown in FIG. The control device 18 of the hologram recording / reproducing apparatus performs individual control such as opening / closing of the shutters 2 and 9, display control of the data page to the spatial modulator (SLM) 14, and rotation control of the angle rotation mirror 8, and control of the entire apparatus. .

  Next, the operation of this embodiment will be described. When recording data on the hologram recording medium 60, the data page to be recorded is displayed on the spatial modulator (transmissive liquid crystal display device) 14 with the shutter 2 closed (shutter 9 opened), and After the hologram recording medium 60 is rotated and the recording location (recording area) is determined, the shutter 2 is opened.

  As a result, the coherent laser light emitted from the laser light source 1 is incident on the deflecting beam splitter (PBS) 4 after the deflection direction is adjusted by the half-wave plate 3 through the shutter 2 and is referred to the recording light 100. Divided into light 200. The reference light 200 becomes parallel light whose beam diameter is enlarged by the spatial filter 11 and the collimating lens 12, and its traveling direction is changed by the mirror 7 and enters the angle rotation mirror 8. The reference light 200 is applied to the hologram recording medium 60 by the angle rotation mirror 8 at an incident angle set. However, the incident angle of the reference beam 200 to the hologram recording medium 60 is changed by changing the rotation angle of the angle rotation mirror 8 in accordance with a control command from the control device 18. At that time, the angular pitch of the reference beam 200 is scheduled according to the change in the incident angle of the reference beam 200, and the rotation angle of the angle rotation mirror 8 is changed according to this scheduling.

  On the other hand, the signal light 100 is incident on the mirror 10 via the shutter 9, where the path is changed, and then becomes parallel light expanded to a predetermined beam diameter by the spatial filter 11 and the collimating lens 12. The signal light 100 is further incident on the mirror 13, its route is changed, and is incident on the spatial modulator 14. A data page is displayed on the spatial modulator 14 and is spatially modulated by the signal light 100 passing therethrough. The spatially modulated signal light 100 is collected in the hologram recording medium 60 via the signal light lens 15 and overlaps with the reference light 200, and the resulting light intensity distribution of interference fringes is generated in the hologram recording medium 60 as a hologram. After recording, the shutter 2 is closed. The reference light 200 and the signal light 100 irradiated in the hologram recording medium 60 interfere in the hologram recording medium 60, and the light intensity distribution of the resulting interference fringes is recorded in the recording medium as a hologram.

  After that, the data page to be recorded next is displayed on the spatial modulator 14, and the angle rotating mirror 8 is slightly rotated to change the incident angle of the reference beam 200. Then, when the shutter 2 is opened, the next recording is performed. The data page to be recorded is recorded in the same recording area of the hologram recording medium 60 by angle multiplexing. By repeating this, when the predetermined multiplicity is reached, the hologram recording medium 60 is moved and the same multi-recording as described above is performed in the next recording area.

  When reproducing the hologram recorded as described above, the reproduction reference beam 200 is irradiated to the position where the hologram is recorded by opening the shutter 2 with the signal light shutter 9 closed. The reproduction light 300 generated by the irradiation of the reproduction reference light 200 is imaged on the imaging device 17 via the reproduction light lens 16, and a reproduction image signal is obtained from the imaging device 17. Next, the angle rotation mirror 8 is rotated to change the incident angle of the reference light 200 to the hologram recording medium 60 to generate the reproduction light 300 corresponding to another data page from the same recording area. Is obtained. Note that the angle pitch of the reproduction reference beam 200 is scheduled according to the change in the incident angle of the reproduction reference beam 200, and the rotation angle of the angle rotation mirror 8 is changed according to this scheduling, as in the recording.

  Next, control for scheduling the angular pitch of the reference light 200 and changing the angular pitch for each incident angle of the reference light 200 will be described. When recording m data pages in one recording area, the control device 18 of the recording apparatus of this example performs scheduling as shown in the table of FIG. 3 so that the angular pitch of the reference beam 200 is different in the multiple recording order. . Note that the angle control according to the reference light incident angle sequence shown in the table of FIG. 3 is referred to as angle scheduling. In this case, the movable range θtotal of the reference light reflecting mirror of the angle multiplexing system is as follows.

θtotal = Δθ ₁ + Δθ ₂ + ... + Δθ _m-1 (4)

  That is, the angular pitch of this embodiment is not constant as in the prior art, but is determined as shown in FIG. 4 and changes according to the incident angle of the reference light 200. For each incident angle of the reference light 200, the angle pitch of the minimum reference light that ensures angle selectivity is set. This can be said to be the minimum angular pitch in a range where the crosstalk does not deteriorate. Here, when Δθm obtained according to the change in the incident angle of the reference beam 200 according to Equation 2 is an angle pitch, the following relationship is established.

Δθ = Δθ ₁ > Δθ ₂ > Δθ ₃ >……> Δθ _m-1 > Δθ _m (5)

From the above discussion, when the range of motion of the mirror is compared between the conventional method and the method of this embodiment, there is a relationship of θtotal (conventional)> θtotal (invention), and the conventional method is used when the same number of data pages are multiplexed. In contrast, the angle multiplexing method according to the present invention requires a narrower movable range of the mirror. On the contrary, in the same mirror movable range, the method of the present embodiment can increase the multiplicity, and as a result, the recording density can be improved. In the angle scheduling, the incident angle series of the reference light corresponding to the recording from the data page 1 to the data page m is {θ _m , θ _m−1 even in the order of {θ ₁ , θ ₂ ,..., Θ _m }. ,..., Θ ₁ }.

(Snell's law)
The angle selectivity of Bragg in the recording material is expressed as follows, _assuming that θ _o = θ _r = θ _inside in Equation 2.

Δθ _inside = λ / (2Tsin θ _inside ) (6)

  In the angle multiplexing recording apparatus, the reference light is refracted according to Snell's law when entering the recording material.

As shown in FIG. 5, when the refractive index in the recording material is n (1 in the air), the incident angle of light in the recording material is θ _inside , and the incident angle in the air is θ _outside , the following _expression is obtained. Is done.

nsinθ _inside = sinθ _outside (7)

  Since the change in the incident angle of the reference light is obtained by the rotation of a mirror disposed in the apparatus (in the air), the angle pitch needs to be scheduled according to the angle selectivity Δθoutside in the air.

Δθ _outside = dθ _outside / dθ _inside Δθ _inside (8)

That is, a series of Δθ _outside obtained by calculating Expressions 7 and 8 based on the reference light incident angle θ in each step of multiple recording may be used as angle scheduling.

  A case will be described in which the data haze is reproduced by recording the hologram recording medium 60 as described above by angle multiplexing. Since the reproduction reference beam 200 at the time of reproduction must have the same incident angle as the reference beam at the time of recording, the angle pitch changes according to the incident angle of the reference beam as in the case of recording, and is similar to FIG. It becomes scheduling.

  However, there are cases where the shrinkage of the hologram recording medium 60 must be taken into consideration during reproduction. Generally, a photopolymer shrinks due to polymerization of monomers in a material. In order to correct the angle of this contraction, when it is necessary to reproduce the recording light by shifting it by δθ from the angle of the reference light at the time of recording, the angle scheduling of the reproduction reference light is as shown in the table of FIG. Note that since the shrinkage is started during recording, the shrinkage correction amount δθ may differ depending on the data page. In that case, angle scheduling may be performed based on the different δθ.

  According to the present embodiment, the angle pitch of the reference light incident angle is set to an optimum value according to the reference light incident angle at the time of recording by the angle multiplexing method, for example, the minimum reference light angle pitch that ensures angle selectivity, By performing multiple recording by changing the angle pitch for each reference light incident angle, if the multiplicity is the same as before, the range of motion of the mirror can be made narrower than the conventional method without deteriorating crosstalk. The optical components of the reference light optical system can be miniaturized, and the angle rotating mirror 8 can be miniaturized. In addition, since the amount of change in the incident angle of the reference beam 200 is reduced, the change in the power density of the reference beam 200 is also reduced, so that the change in the recording / reproducing conditions itself is also reduced, thereby simplifying the apparatus. In addition, when angle scheduling is performed, angle correction suitable for reproduction of each data page can be performed even if the recording material contracts, so that data reproduction with better S / N is possible. On the contrary, if the mirror movable range is the same as the conventional one, the multiplicity can be increased more than the conventional one without deteriorating the crosstalk, and the recording density can be improved.

  In addition, according to this embodiment, this invention is not limited to the said embodiment, In the range which does not deviate from the summary, in a concrete structure, a function, an effect | action, and an effect also by other various forms. Can be implemented.

It is the block diagram which showed the structure of the hologram recording / reproducing apparatus which concerns on one embodiment of this invention. FIG. 2 is a block diagram showing a configuration of a control system of the hologram recording / reproducing apparatus of the present embodiment shown in FIG. 1. It is the figure which showed the relationship between the recording order of the data page to carry out multiple recording, and a reference beam incident angle. It is the characteristic view which showed the relationship between the incident angle of reference light, and an angle pitch. It is a figure explaining Snell's law. It is the figure which showed the relationship between the reproduction | regeneration order of the data page recorded by multiple recording, and the reference light incident angle. FIG. 6 is a characteristic diagram showing a relationship between diffraction efficiency and an incident angle change amount of reproduction reference light. It is the characteristic view which showed the relationship between the incident angle of reference light, and an angle pitch. It is the figure which showed the relationship between the recording order of the conventional data page to carry out multiple recording, and a reference light incident angle.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Laser light source, 2, 9 ... Shutter, 3 ... Half wave plate, 4 ... Deflection beam splitter (PBS), 5, 11 ... Spatial filter, 6, 12 ... Collimating lens, 7, 10 , 13 ... mirror, 8 ... angle rotating mirror, 14 ... spatial modulator (SLM), 15 ... signal light lens, 16 ... reproduction light lens, 17 ... imaging device, 18 ... control device, 60 ...... Hologram recording medium.

Claims (9)

When recording interference fringes of reference light and signal light on a hologram recording medium, the hologram recording apparatus performs multiplex recording of data in the same recording area by changing an incident angle of the reference light to the hologram recording medium. ,
Reference light incident angle control means for changing the angle pitch of the incident angle of the reference light during the multiple recording,
A hologram recording apparatus characterized by that.   2. The hologram recording apparatus according to claim 1, wherein the reference light incident angle control means changes the angle pitch of the reference light incident angles in accordance with the reference light incident angle.   When the angle pitch of the incident angle of the reference light is set according to the incident angle of the reference light, the angle based on the refractive index of the air in which the reference light travels and the hologram recording medium and the incident angle. The hologram recording apparatus according to claim 1, wherein the pitch is corrected. A hologram reproducing apparatus for reproducing multiplexed recording data by changing the incident angle of the reproduction reference light to the hologram recording medium and irradiating the same area of the hologram recording medium,
Reproduction reference light incident angle control means for changing the angle pitch so that the incident angle of the reproduction reference light is the same as the incident angle of the reference light at the time of recording,
A hologram reproducing apparatus characterized by that.   5. The hologram reproducing apparatus according to claim 4, wherein when the incident angle of the reproduction reference light to the hologram recording medium is set, the incident angle is corrected according to a volume change of the hologram recording medium. A holographic recording method for multiplex recording data in the same recording area by changing an incident angle of the reference light to the hologram recording medium when recording interference fringes of reference light and signal light on a hologram recording medium. ,
A hologram recording method, wherein an angle pitch of an incident angle of the reference light at the time of the multiplex recording is changed.   The hologram recording method according to claim 6, wherein an angle pitch of an incident angle of the reference light is changed according to an incident angle of the reference light. A hologram reproduction method for reproducing multiplexed recording data by irradiating the same area of the hologram recording medium by changing the incident angle of the reproduction reference light to the hologram recording medium,
A hologram reproducing method, wherein the angle pitch is changed so that the incident angle of the reproduction reference light is the same as the incident angle of the reference light at the time of recording.   9. The hologram reproducing method according to claim 8, wherein when the incident angle of the reproduction reference light to the hologram recording medium is set, the incident angle is corrected in accordance with a volume change of the hologram recording medium.

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